Semiconductor helices offer control over light in a format that could work in the pixels of displays. Here's how researchers managed to twist semiconductor material.
Amino acids are the quintessential building blocks of proteins," said Wenchun Feng, a postdoctoral research fellow in Kotov's lab and lead author of a newly published paper on the work. "The direction of the spiral of proteins is determined by the geometrical property of amino acids. We found that a common amino acid, cysteine, working together in large numbers, can twist not only proteins but also semiconductors."
The team from University of Michigan coated nanoparticles made out of cadmium telluride, a semiconductor capable of emitting light, with cysteine. Cysteine comes in two forms that are mirror images of one another like our hands are, so it is known as a "chiral" molecule. They observed the nanoparticles spontaneously self-assembling into semiconductor "tornadoes" following the rightward or leftward chirality of the amino acid.
The scientists were surprised by the high fidelity of this self-assembly process and the strength of the twist. Nearly all—98%—of semiconductor helices had the same twisting direction and indeed looked like nanoscale fusilli. Some organic molecules can form natural spirals, too, but the light-twisting ability of semiconductor helices made by Kotov and colleagues is at least five times stronger.
When they shone light through the semiconductors, they recorded the photons swirling through them. Through a combination of experiments and computer simulations, the researchers developed design principles and methods for engineering the optical properties of the semiconductor helices for the different colors in future holography devices.
Figure 2: Colourised electron microscope images of a right-handed helix and a left-handed helix. (Source: Wenchun Feng, Kotov Lab)
One of the unexpected consequences of this technology-driven project was getting a peek into mysteries surrounding how life may have arisen on Earth and why many biological molecules reliably follow either a clockwise or counterclockwise spiral. Kotov suggests that amino acids, which are known to form spontaneously in space dust, may have assembled nanoparticles into spirals that twisted the light from the early stars, serving as stable inorganic templates for organic molecules and particles to follow the same pattern.